193nm laser and inspection system

ABSTRACT

An improved solid-state laser for generating sub-200 nm light is described. This laser uses a fundamental wavelength between about 1030 nm and 1065 nm to generate the sub-200 nm light. The final frequency conversion stage of the laser creates the sub-200 nm light by mixing a wavelength of approximately 1109 nm with a wavelength of approximately 234 nm. By proper selection of non-linear media, such mixing can be achieved by nearly non-critical phase matching. This mixing results in high conversion efficiency, good stability, and high reliability.

PRIORITY APPLICATION

The present application is a divisional of U.S. patent application Ser.No. 14/170,384, filed Jan. 31, 2014, which claims priority to U.S.Provisional Patent Application 61/764,441, filed on Feb. 13, 2013 andincorporated by reference herein.

RELATED APPLICATIONS

The present application is related to U.S. Published Application2013/0313440 entitled “Solid-state 193 nm laser and an Inspection Systemusing a Solid-State 193 nm laser”, by Chuang et al. and published Nov.28, 2013, which is incorporated by reference herein. This application isalso related to U.S. Pat. No. 8,755,417, entitled “Coherent lightgeneration below about 200 nm”, by Dribinski et al. and issued Jun. 17,2014, PCT Published Application WO2012/154468 by Lei et al. andpublished Nov. 15, 2012, U.S. Provisional Application 61/538,353,entitled “Solid-State 193 nm Laser And An Inspection System Using ASolid-State 193 nm Laser”, by Chuang et al. and filed Sep. 23, 2011,U.S. Provisional Application 61/559,292 entitled “Solid-State 193 nmLaser And An Inspection System Using A Solid-State 193 nm Laser”, byChuang et al. and filed Nov. 14, 2011, U.S. Provisional Application61/591,384, entitled “Solid-State 193 nm Laser And An Inspection SystemUsing A Solid-State 193 nm Laser”, by Chuang et al. and filed Jan. 27,2012, U.S. Provisional Application 61/603,911, entitled “Solid-State 193nm Laser And An Inspection System Using A Solid-State 193 nm Laser”,Chuang et al. and filed Feb. 27, 2012, U.S. Published Application2013/0077086, entitled “193 nm Laser and Inspection System using 193 nmLaser”, by Chuang et al. and published Mar. 28, 2013, U.S. ProvisionalApplication 61/666,675 entitled “Scan rate for Continuous Motion of aCrystal in a Frequency Converted Laser”, by Armstrong and filed Jun. 29,2012, U.S. Pat. No. 9,042,006 entitled “Solid State Illumination SourceAnd Inspection System”, by Armstrong and issued May 26, 2015, and U.S.Pat. No. 8,929,406 entitled “193 nm Laser and Inspection System” byChuang et al. and Jan. 6, 2015. All of the above applications areincorporated by reference herein.

BACKGROUND OF THE DISCLOSURE

Field of the Disclosure

The present application relates to a solid-state laser that generateslight near 193 nm and is suitable for use in photomask, reticle, orwafer inspection.

Related Art

The integrated circuit industry requires inspection tools withincreasingly higher resolution to resolve ever smaller features ofintegrated circuits, photomasks, solar cells, charge coupled devicesetc., as well as detect defects whose sizes are of the order of, orsmaller than, feature sizes. Short wavelength light sources, e.g.sources generating light under 200 nm, can provide such resolution.Specifically for photomask or reticle inspection, it is desirable toinspect using a wavelength identical, or close, to the wavelength thatwill be used for lithography, e.g. substantially 193.368 nm, as thephase-shifts of the inspection light caused by the patterns will beidentical or very similar to those caused by the same patterns duringlithography. However, the light sources capable of providing such shortwavelength light are practically limited to excimer lasers and a smallnumber of solid-state and fiber lasers. Unfortunately, each of theselasers has significant disadvantages.

An excimer laser generates an ultraviolet light, which is commonly usedin the production of integrated circuits. An excimer laser typicallyuses a combination of a noble gas and a reactive gas under high pressureconditions to generate the ultraviolet light. A conventional excimerlaser generating 193 nm wavelength light, which is increasingly a highlydesirable wavelength in the integrated circuit industry, uses argon (asthe noble gas) and fluorine (as the reactive gas). Unfortunately,fluorine is toxic and corrosive, thereby resulting in high cost ofownership. Moreover, such lasers are not well suited to inspectionapplications because of their low repetition rate (typically from about100 Hz to several kHz) and very high peak power that could result indamage of samples during inspection. Furthermore, high-speed inspectiontypically requires minimum laser pulse repetition rates of multiple MHz(e.g. greater than 50 MHz in some cases) in order to allow high-speedimage or data acquisition with low noise.

A small number of solid state and fiber based lasers producing sub-200nm output are known in the art. Unfortunately, most of these lasers havevery low power output (e.g. under 60 mW), or very complex design, suchas two different fundamental sources or eighth harmonic generation, bothof which are complex, unstable, expensive and/or commerciallyunattractive.

Therefore, a need arises for a laser capable of generating 193 nm lightyet overcoming the above disadvantages.

SUMMARY OF THE DISCLOSURE

A laser for generating ultraviolet light with a vacuum wavelength near193 nm, such as in a wavelength range between 190 nm and 200 nm, isdescribed. This laser includes a fundamental source and multiple stagesfor generating harmonic, sum and other frequencies. In preferredembodiments, the fundamental source can generate a fundamental frequencycorresponding to a wavelength of approximately 1064 nm to 1065 nm. Inother embodiments, the fundamental can generate a wavelength ofapproximately 1053 nm or approximately 1047 nm. Fundamental wavelengthsin the range from about 1047 nm to 1065 nm can be used in one or moreembodiments of the sub-200-nm laser described herein. Lasers that cangenerate wavelengths in this range include Yb-doped fiber lasers, Nd:YAGlasers (neodymium-doped yttrium aluminum garnate), neodymium-dopedyttrium orthovanadate lasers, and Nd:YLF (neodymium-doped yttriumlithium fluoride) lasers. Where a wavelength value without qualificationis given in this specification, it is to be assumed that wavelengthvalue refers to the wavelength in vacuum.

A first stage uses a portion of the fundamental frequency to generate awavelength of approximately 1109 nm. In one embodiment a fiber is usedto generate or amplify light at a wavelength of approximately 1109 nmfrom a portion of the fundamental wavelength. In a second embodiment ofthis stage, an OPO or OPA is used to generate or amplify a wavelengthnear 2218 nm from a portion of the fundamental. In this secondembodiment, the wavelength near 2218 nm is frequency-doubled to createlight at a wavelength of approximately 1109 nm.

In one embodiment a second stage can generate a second harmonicfrequency from a portion of the fundamental frequency. Generating asecond harmonic of a wavelength near 1064 nm, 1053 nm or 1047 nm is wellknown. Several different non-linear crystals can be used to do this,including, but not limited to, KTP (potassium titanyl phosphate), KDP(potassium dihydrogen phosphate), KBBF (potassiumfluoroboratoberyllate), CBO (cesium triborate), CLBO (cesium lithiumborate), BBO (beta barium borate), LBO (lithium triborate) and LB4(lithium tetraborate). A third stage generates a wavelength ofapproximately 234 nm from another portion of the fundamental and secondharmonic. Apparatus and methods for generating a wavelength ofapproximately 234 nm from the fundamental and the second harmonic aredescribed below.

In an alternative embodiment a second stage can generate a wavelength ofapproximately 1171 nm from a portion of the fundamental frequency, orfrom a portion of the approximately 1109 nm wavelength light. A thirdstage generates the fifth harmonic of the approximately 1171 nmwavelength in order to create a wavelength of approximately 234 nm.

In the above described embodiments, a fourth stage combines thewavelength near 234 nm with the wavelength near 1109 nm to generate awavelength near 193 nm. In some embodiments the wavelength generated inthe fourth stage may be substantially 193.4 nm. In some preferredembodiments this frequency combination may be achieved using nearnon-critical phase matching in a CLBO crystal (the phase matching angleis approximately 85° at a temperature near 120° C.). This results ingood conversion efficiency, low walk-off and good stability. In someembodiments, BBO may be used instead of CLBO. For type I mixing in BBO,the phase matching angle is approximately 57° at a temperature near 120°C., the walk-off is larger than for CLBO (about 98 mrad compared withabout 7 mrad), but d_(eff) is about 70% larger than for CLBO (about 1.9pm V⁻¹ compared with about 1.1 pm V⁻¹). Type II mixing in BBO is alsopossible at a phase matching angle of about 63°, with a lower d_(eff)(approximately 0.6 pm V¹) and a walk-off angle of about 85 mrad. SinceCLBO and BBO are hygroscopic materials, in one embodiment the crystal isoperated at a temperature around 120° C. or higher to prevent absorptionof water from the environment. In another embodiment, the crystal iskept protected from humidity, for example by enclosing the crystal in apurged low-humidity environment, and the crystal is operated at a lowertemperature, such as one near 100° C., 80° C. or 50° C. When the crystaloperating temperature is different from 120° C., an appropriate changemust be made to the phase-matching angle. In some preferred embodiments,the non-linear crystal used in this and other frequency-conversionstages is a hydrogen-annealed crystal as described in co-pending U.S.Pat. No. 9,250,178 and issued on Feb. 2, 2016 by Chuang et al, andclaiming priority to U.S. Provisional Application 61/544,425 filed onOct. 7, 2011. Both of which are incorporated by reference herein.

In one embodiment, the third stage can combine a portion of the secondharmonic frequency with a portion of the fundamental to generate a thirdharmonic frequency. In this embodiment, the third stage uses anotherportion of the second harmonic to generate or amplify a wavelength near689 nm using an OPO or OPA. This embodiment of the third stage combinesthe third harmonic frequency and the wavelength near 689 nm to generatea sum frequency corresponding to a wavelength of approximately 234 nm.In some embodiments, the combination of the third harmonic and thewavelength near 689 nm is done using a CLBO crystal. At a temperaturenear 120° C. the phase matching angle is approximately 75°, d_(eff) isabout 0.9 pm V⁻¹, and the walk-off angle is about 20 mrad. In otherembodiments, the combination of the third harmonic and the wavelengthnear 689 nm is done using a BBO crystal. At a temperature near 120° C.,the phase matching angle is approximately 55°, d_(eff) about 1.6 pm V⁻¹,and the walk-off angle is about 85 mrad.

In an alternative embodiment, the third stage generates a fourthharmonic frequency from the second harmonic frequency. In thisembodiment, the third stage uses a portion of the fundamental togenerate or amplify a wavelength near 1954 nm using an OPO or OPA. Thisembodiment of the third stage combines the wavelength near 1954 nm withthe fourth harmonic to generate a wavelength near 234 nm. In someembodiments, the combination of the fourth harmonic and the wavelengthnear 1954 nm is done using an LBO crystal, an LB4 crystal, a CLBOcrystal or a BBO crystal.

In another embodiment, the third stage generates a fifth harmonicfrequency from a wavelength of approximately 1171 nm. The fifth harmonicof a wavelength of near 1171 nm has a wavelength near 234 nm. In someembodiments, the approximately 234 nm wavelength has a wavelength ofsubstantially 234.2 nm. The fifth harmonic of the wavelength near 1171nm is created by first creating a second harmonic from a portion of thelight at a wavelength near 1171 nm. This may be done, for example, usingLBO, which is phase matched at an angle of about 83° for a temperaturenear 120° C., has a d_(eff) of about 0.8 pm V⁻¹, and has a low walk-offof about 6 mrad. In one embodiment, the second harmonic is converted toa fourth harmonic, and the fourth harmonic is combined with a portion ofthe light at 1171 nm to create a fifth harmonic. In another embodiment,a portion of the second harmonic harmonic is combined with a portion ofthe light at a wavelength near 1171 nm to create a third harmonic, thenthe third harmonic is combined with a portion of the second harmonic tocreate a fifth harmonic. Non-linear crystals such as CLBO and BBO aresuitable for creating the third, fourth and fifth harmonics of awavelength 1171 nm. Other non-linear materials such as LB4 may besuitable for some of the conversion steps.

In some embodiments, the second stage generates a wavelength ofapproximately 1171 nm from a portion of the fundamental. In oneembodiment, a portion of the light at the wavelength near 1109 nm isshifted to a wavelength near 1171 nm by first-order Raman shift. Thefirst-order Raman shift gain has a broad peak near 440 cm⁻¹, so thesecond-order Raman shift is very effective at shifting a wavelength near1109 nm to a wavelength near 1171 nm. In another embodiment, thewavelength of approximately 1171 nm is generated by second-order Ramanscattering of a portion of the fundamental wavelength. The second-orderRaman shift gain has a broad peak near 880 cm⁻¹, so the second-orderRaman shift can be effective at shifting a fundamental near 1064 nm ornear 1053 nm to a wavelength near 1171 nm.

In another embodiment, the laser can also include an optical amplifierfor amplifying the fundamental frequency.

A method of generating light with a wavelength between about 190 nm and200 nm, such as a wavelength of approximately 193 nm, is also described.This method includes generating a fundamental frequency of approximately1064 nm, approximately 1053 nm or approximately 1047 nm. A portion ofthe fundamental frequency can be used to generate a wavelength ofapproximately 1109 nm. Another portion of the fundamental frequency canbe used to generate a second harmonic frequency. Another portion of thefundamental frequency can be combined with the second harmonic frequencyto generate a wavelength of approximately 234 nm. The approximately 1109nm wavelength and the approximately 234 nm can be combined to generate awavelength of approximately 193.4 nm.

An alternative method of generating approximately 193 nm wavelengthlight is also described. This method includes generating a fundamentalfrequency of approximately 1064 nm, approximately 1053 nm orapproximately 1047 nm. A portion of the fundamental frequency can beused to generate a wavelength of approximately 1109 nm. Another portionof the fundamental frequency can be used to generate a wavelength ofapproximately 1171 nm. The wavelength of approximately 1171 nm can beconverted to its fifth harmonic at a wavelength of approximately 234 nm.The approximately 1109 nm wavelength and the approximately 234 nm can becombined to generate a wavelength of approximately 193.4 nm.

A pulse multiplier is also described. This pulse multiplier includes alaser system for generating a regular series of input laser pulses. Thelaser system can include a light source at approximately 1064 nm, 1053nm or 1047 nm and frequency conversion stages generating the input laserpulses at approximately 193 nm. A beam splitter can receive the inputlaser pulses. A set of mirrors can create a ring cavity including thebeam splitter, wherein the beam splitter directs a part of, orsubstantially all of, each input pulse into the ring cavity, and whereinthe beam splitter further directs a fraction of each pulse out of thering each time that pulse traverses the ring.

An inspection system incorporating a 193 nm laser and a coherencereducing subsystem comprising a dispersive element and/or anelectro-optic modulator is also described.

An optical inspection system for inspecting a surface of a photomask,reticle, or semiconductor wafer for defects is also described. Thissystem can include a laser system for generating a beam of radiation ata wavelength between about 190 nm and 200 nm. This laser system caninclude a generator for generating a wavelength near 1109 nm that isused to create the sub-200-nm beam of radiation. The laser system mayfurther include an annealed crystal and a housing to maintain theannealed condition of the crystal. The light reflected or scattered fromthe article being inspected is used to determine the presence ofdefects. In some embodiments, both transmitted and reflected light arecollected and are used together for determining the presence of defects.In some embodiments, the transmitted and reflected light are collectedon the same detector to ensure proper registration between the two setsof data.

An inspection system for inspecting a surface of a sample is alsodescribed. This inspection system includes an illumination subsystemconfigured to produce a plurality of channels of light, each channel oflight produced having differing characteristics from at least one otherchannel of light energy. The illumination subsystem includes a laser forgenerating 193 nm wavelength light for at least one channel. Optics areconfigured to receive the plurality of channels of light and combine theplurality of channels of light energy into a spatially separatedcombined light beam and direct the spatially separated combined lightbeam toward the sample. A data acquisition subsystem includes at leastone detector configured to detect reflected light from the sample. Thedata acquisition subsystem can be configured to separate the reflectedlight into a plurality of received channels corresponding to theplurality of channels of light.

A catadioptric imaging system with dark-field illumination is alsodescribed. This system can include an ultraviolet (UV) light source forgenerating UV light. This UV light source can include a laser system forgenerating a beam of radiation at a wavelength between about 190 nm and200 nm. This laser system can include a generator for generating awavelength near 1109 nm that is used to create the sub-200-nm beam ofradiation. The laser system may further include an annealed crystal anda housing to maintain the annealed condition of the crystal. Adaptationoptics are also provided to control the illumination beam size andprofile on the surface being inspected. The catadioptric imaging systemalso includes a catadioptric objective, a focusing lens group, and azooming tube lens section in operative relation to each other. A prismcan be provided for directing the UV light along the optical axis atnormal incidence to a surface of a sample and directing specularreflections from surface features of the sample as well as reflectionsfrom optical surfaces of the objective along an optical path to animaging plane.

A surface inspection apparatus is also described. This apparatus caninclude a laser system for generating a beam of radiation at awavelength between about 190 nm and 200 nm. This laser system caninclude a generator for generating a wavelength near 1109 nm that isused to create the sub-200-nm beam of radiation. The laser system mayfurther include an annealed crystal and a housing to maintain theannealed condition of the crystal. An illumination system can beconfigured to focus the beam of radiation at a non-normal incidenceangle relative to a surface to form an illumination line on the surfacesubstantially in a plane of incidence of the focused beam. The plane ofincidence is defined by the focused beam and a direction that is throughthe focused beam and normal to the surface.

A collection system can be configured to image the illumination line. Inone embodiment, the collection system can include an imaging lens forcollecting light scattered from a region of the surface comprising theillumination line. A focusing lens can be provided for focusing thecollected light. A device including an array of light sensitive elementscan also be provided. In this array, each light sensitive element of thearray of light sensitive elements can be configured to detect acorresponding portion of a magnified image of the illumination line.

An optical system for detecting anomalies of a sample is also described.This optical system includes a laser system for generating sub-200-nmwavelength light. The laser system includes a light source, an annealed,frequency-conversion crystal, a housing, and beam shaping optics. Thehousing is provided to maintain an annealed condition of the crystal.The beam shaping optics can be configured to receive a beam from thelight source and focus the beam to an elliptical cross section at a beamwaist in or proximate to the crystal.

First optics can direct a first beam of radiation along a first pathonto a first spot on a surface of the sample. In some embodiments,second optics can direct a second beam of radiation along a second pathonto a second spot on a surface of the sample. The first and secondpaths are at different angles of incidence to the surface of the sample.Collection optics can include a curved mirrored surface that receivesscattered radiation from the first or the second spot on the samplesurface and originating from the first or second beam. The collectionoptics focuses the scattered radiation to a first detector. The firstdetector provides a single output value in response to the radiationfocused onto it by said curved mirrored surface. An instrument can beprovided that causes relative motion between the sample and the firstand second beams so that the spots are scanned across the surface of thesample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a block diagram of an exemplary laser for generating193 nm light using a fundamental wavelength near 1064 nm, 1053 nm, or1047 nm.

FIG. 1B illustrates a block diagram of an alternative exemplary laserfor generating 193 nm light using a fundamental wavelength near 1064 nm,1053 nm, or 1047 nm.

FIG. 1C illustrates a block diagram of another alternative exemplarylaser for generating 193 nm light using a fundamental wavelength near1064 nm or 1053 nm.

FIG. 2A illustrates a block diagram of one exemplary generator forgenerating a wavelength of approximately 1109 nm.

FIG. 2B illustrates a block diagram of an alternative exemplarygenerator for generating a wavelength of approximately 1109 nm.

FIG. 2C illustrates a block diagram of another alternative exemplarygenerator for generating a wavelength of approximately 1109 nm.

FIG. 3 illustrates a block diagram of an exemplary frequency mixer forgenerating 193 nm light by mixing a wavelength near 1109 nm with awavelength near 234 nm.

FIG. 4A illustrates a block diagram of an exemplary generator thatgenerates a wavelength of approximately 234 nm from the fundamental andsecond harmonic.

FIG. 4B illustrates a block diagram of an alternative exemplarygenerator that generates a wavelength of approximately 234 nm from thefundamental and second harmonic.

FIG. 5A illustrates an exemplary generator for generating a wavelengthnear 1171 nm.

FIG. 5B illustrates an alternative exemplary generator for generating awavelength near 1171 nm.

FIG. 6A illustrates an exemplary 5^(th) harmonic generator forgenerating the 5^(th) harmonic of a wavelength of approximately 1171 nm.

FIG. 6B illustrates an alternative exemplary 5^(th) harmonic generatorfor generating the 5^(th) harmonic of a wavelength of approximately 1171nm.

FIG. 7 illustrates an exemplary embodiment of the fundamental laser.

FIG. 8 illustrates an exemplary pulse multiplier that may be used incombination with the sub-200 nm laser and an inspection or metrologysystem.

FIG. 9 illustrates an exemplary coherence reducing subsystem that may beused in combination with the sub-200 nm laser and an inspection ormetrology system.

FIG. 10 illustrates an exemplary inspection system including the sub-200nm laser.

FIG. 11 illustrates an exemplary inspection system including multipleobjectives and the sub-200 nm laser.

FIG. 12 illustrates an exemplary inspection system with dark-field andbright-field modes and including the sub-200 nm laser.

FIGS. 13A and 13B illustrate an exemplary dark-field patterned-waferinspection system including the sub-200 nm laser.

FIG. 14 illustrates an exemplary unpatterned-wafer inspection systemincluding the sub-200 nm laser.

FIG. 15 illustrates another exemplary unpatterned-wafer inspectionsystem including the sub-200 nm laser.

DETAILED DESCRIPTION OF THE DRAWINGS

An improved laser for generating light with a wavelength near 193 nm,such as a wavelength in the range from 190 nm to 200 nm, is described.FIG. 1A illustrates a simplified block diagram of an exemplaryembodiment of a laser 100 for generating 193 nm light. This laser 100generates the output wavelength near 193 nm by mixing a wavelength ofapproximately 1109 nm with a wavelength of approximately 234 nm. Theapproximately 1109 nm light and the approximately 234 nm light aregenerated from the same fundamental laser.

In one embodiment, laser 100 includes a fundamental laser 102 operatingat a wavelength near 1064 nm, which generates a fundamental light 101 atfrequency ω. In other embodiments, other wavelengths such as 1047 nm or1053 nm can be used for the fundamental laser 102. The fundamental laser102 may be a fiber laser, or may be based on Nd:YAG, Nd-doped yttriumorthovanadate or Nd:YLF. The fundamental laser 102 is preferably apulsed laser, such as a mode-locked laser or a Q-switched laser.

A second harmonic generator 104 creates the second harmonic 2ω of thefundamental. The second harmonic generator 104 outputs a light 103 thatincludes the second harmonic 2ω and a part of the fundamental ω that isnot consumed in the second harmonic generation process. The light 103from the second harmonic generator 104 is directed to frequencyconversion stages 106.

With the light 103 (i.e. from the fundamental ω and the second harmonic2ω), the frequency conversion stages 106 generate a light 107 having awavelength near 234 nm, such as a wavelength of substantially 234.2 nm.Frequency conversion stages 106 also output a light 105 including theunconsumed fundamental (ω). Exemplary embodiments of frequencyconversion stages 106 are described below.

A 1109 nm generator 108 generates a wavelength near 1109 nm from aportion of the light 105 at the fundamental frequency ω. Although FIG.1A shows that the light 105 is output by the frequency conversion stages106, in other embodiments (not shown) that unconsumed fundamental couldbe taken directly from the fundamental laser 102 or from the output ofthe second harmonic generator 104. In yet other embodiments, not shown,the unconsumed fundamental from the output of the 1109 nm generator 108is directed to the second harmonic generator 104 and/or the frequencyconversion stages 106. There are many different ways to direct thefundamental between the second harmonic generator 104, the frequencygenerator 106, and the 1109 nm generator 108. All such different schemesare within the scope of the present invention. Exemplary embodiments ofthe 1109 nm generator are described below.

A frequency mixer 110 generates the laser output having a wavelengthnear 193 nm by mixing the light 109 having a wavelength of approximately1109 nm with the light 107 having a wavelength of approximately 234 nm.This mixing is nearly non-critically phase matched in CLBO at atemperature near 80-120° C. Notably, this mixing results in goodconversion efficiency, low walk-off and good stability. Even lowertemperatures, such as about 30-80° C. result in good conversionefficiency, low walk-off and acceptable stability and may be used insome embodiments. In some embodiments, BBO may be used instead of CLBO.

FIG. 1B illustrates a simplified block diagram of an alternativeembodiment of a laser 120 for generating 193 nm light. In thisembodiment, laser 120 includes a fundamental laser 122 operating at awavelength near 1064 nm, which generates a fundamental light 121 atfrequency ω. As described above, other wavelengths such as 1047 nm or1053 nm can be used for the fundamental laser 122. The fundamental laser122 may be a fiber laser, or may be based on Nd:YAG, Nd-dopedorthovandate or Nd:YLF. The fundamental laser 122 is preferably a pulsedlaser, such as a mode-locked laser or a Q-switched laser.

A 1109 nm generator 128 generates a light 129 having a wavelength near1109 nm from the fundamental light 121. A frequency mixer 110 generatesthe laser output having a wavelength near 193 nm by mixing the light 129having a wavelength of approximately 1109 nm with a light 127 having awavelength of approximately 234 nm. This mixing is nearly non-criticallyphase matched in CLBO at a temperature near 80-120° C. Notably, thismixing results in good conversion efficiency, low walk-off and goodstability. In some embodiments, BBO may be used instead of CLBO.

In this embodiment, a 1171 nm generator 124 creates a light 123 having awavelength near 1171 nm from a portion of a light 129′ at a wavelengthnear 1109 nm. The light 129′ may be taken from unconsumed 1109 nm fromfrequency mixing stage 130 as shown, or may be taken directly from the1109 nm generator 128 (not shown). The 1171 nm generator 124 outputs alight 123 at a wavelength of approximately 1171 nm, which is directed toa fifth harmonic generator 126. The fifth harmonic generator 126generates light near 234 nm, such as a wavelength of substantially 234.2nm, by creating the fifth harmonic of the approximately 1171 nm light.Exemplary embodiments of the 1171 nm generator 124 and thefifth-harmonic generator 126 are described below.

FIG. 1C illustrates a simplified block diagram of another alternativeembodiment of a laser 140 for generating 193 nm light. In thisembodiment, laser 140 includes a fundamental laser 142 operating at awavelength near 1064 nm, which generates a fundamental light 141 atfrequency ω. As described above, other wavelengths such as 1053 nm canbe used for the fundamental laser, and any of the above described lasersmay be used for the fundamental laser 142. The fundamental laser 142 ispreferably a pulsed laser, such as a mode-locked laser or a Q-switchedlaser.

An 1171 nm generator 144 creates a light 143 having a wavelength near1171 nm from a portion of the fundamental light 141. In one embodiment,this portion of the fundamental light 141 may be taken directly from theoutput of the fundamental laser 142. In another embodiment (not shown),an unconsumed fundamental from the 1109 nm generator 148 can be used bythe 1171 nm generator 144. The 1171 nm generator 144 outputs a light 143at a wavelength of approximately 1171 nm. The light 143 is directed to afifth harmonic generator 146 that generates light near 234 nm, such as awavelength of substantially 234.2 nm, by creating the fifth harmonic ofthe approximately 1171 nm light. The fifth harmonic generator 146 mayfunction in a substantially similar manner to the fifth harmonicgenerator 126 (FIG. 1B). Exemplary embodiments of the 1171 nm generator144 and the fifth harmonic generator 146 are described below.

The 1109 nm generator 148 generates a wavelength near 1109 nm from aportion of a fundamental light 145 provided by the fundamental laser142. In some embodiments (not shown), the fundamental light 145 for the1109 nm generator 148 may be taken from an unconsumed fundamental fromthe 1171 nm generator 144. In other embodiments (not shown), theunconsumed fundamental from the 1109 nm generator 148 may be directed tothe 1171 nm generator 144. The 1109 nm generator 148 operatessubstantially similarly to the 1109 nm generators 108 and 128 describedabove. Exemplary embodiments of the 1109 nm generator 148 are describedbelow.

FIG. 2A illustrates a simplified block diagram of an exemplaryembodiment of an 1109 nm generator 200 that can perform the functions ofthe 1109 nm generator 108 (FIG. 1A), the 1109 nm generator 128 (FIG.1B), and the 1109 nm generator 148 (FIG. 1C). In this embodiment, alight 205 at a wavelength of approximately 1109 nm is generated from afundamental light 201 using a Raman amplifier 204. The Raman amplifier204 may include a fused-silica fiber or a germania-doped fused silicafiber. The Raman gain of a fused silica or germania-doped fused silicafiber has a broad peak centered near 440 cm⁻¹ of frequency shift. Theuseful gain extends from a shift of about 300 cm⁻¹ to a shift of about500 cm⁻¹. Any fundamental wavelength between about 1050 nm and about1073 nm is within 300 to 500 cm⁻¹ of 1109 nm, and so such wavelengthsare ideally suited for use as the fundamental wavelength. Wavelengthsjust outside this range (such as 1047 nm) may be useable depending onthe required specification of the output wavelength. A fundamentalwavelength of about 1030 nm could be used with a second-order Ramanshift. The advantage of a germania-doped fiber over undoped fused silicais that the Raman gain is higher, so a shorter length of fiber cansuffice. The advantage of undoped fused silica fiber is that it is lessexpensive and it is not hygroscopic.

The Raman amplifier 204 amplifies the light from an 1109 nm seed laser202. The seed laser 202 is a stable, narrow-band laser that generates alight at the desired wavelength close to 1109 nm. In some preferredembodiments, the output of the seed laser 202 may be between 1 mW and250 mW. In preferred embodiments, the seed laser 202 may be a diodelaser or a fiber laser. Any known technique may be used to stabilize theoutput wavelength of the seed laser 202, such as distributed feedback, afiber-Bragg grating, or an etalon. In preferred embodiments, the Ramanamplifier 204 amplifies the mW-level light from the seed laser 202 tothe 1109 nm light 205 at a power level of between about 1 W and 20 W.

In other embodiments (not shown) of the 1109 nm generator 200, no seedlaser is used. Instead, the Raman amplifier is operated as a Raman laseror oscillator with frequency selective elements incorporated so as tolimit the bandwidth and control the output wavelength.

FIG. 2B illustrates a simplified block diagram of an alternativeexemplary embodiment of an 1109 nm generator 220 that can perform thefunctions of the 1109 nm generator 108 (FIG. 1A) and the 1109 nmgenerator 128 (FIG. 1B). In this embodiment, a 1109 nm light 225 at awavelength is generated from a fundamental light 221 using a non-linearcrystal 228 to generate a light 223 at a wavelength twice equal to twicethe desired wavelength (i.e. a wavelength of approximately 2218 nm),which is then doubled in frequency by a second harmonic generator 238 togenerate the 1109 nm light 225 at the desired wavelength. The secondharmonic generator 238 may use KTP, LNB (lithium niobate), or anothernon-linear crystal.

The fundamental light 221 is focused by a lens 222 and directed into anoptical cavity formed by curved mirrors 224 and 226, a frequencyselector 236, a flat mirror 230, and an output coupler 232. In oneembodiment (shown), the optical cavity further includes a non-linearcrystal 228 comprising a material such as LNB, doped LNB, lithiumtantalate, magnesium-doped lithium tantalate or KTP. In someembodiments, the non-linear crystal 228 may be periodically-poled. Thecurved mirrors 224 and 226 are coated with a coating that is highlyreflective for light with a wavelength near 2218 nm, but issubstantially transparent to wavelengths near the fundamental wavelengthand the idler wavelength which is near 2 pm in wavelength (the exactwavelength depends on the fundamental wavelength, and will typically bein range between about 1980 nm and about 2050 nm). Note that in thisconfiguration, the desired (signal) wavelength is longer than theunwanted (idler) wavelength. The frequency selector 236 is highlyreflective in a narrow band centered on the desired output wavelengthnear 2218 nm, but has high transmission for other wavelengths close tothe desired wavelength. The frequency selector 236 determines thewavelength and bandwidth of the optical parametric oscillator. Inpreferred embodiments, the bandwidth is less than 1 nm, such as a fewtenths of a nanometer. The frequency selector 236 may comprise avolume-Bragg grating, a birefringent filter, a notch filter, or anetalon. The frequency selector 236 may operate in reflection as shown,or a transmissive frequency-selective element may be placed at anappropriate location in the optical cavity with the frequency selector236 acting as a reflector or mirror.

The output coupler 232 transmits a fraction (such as approximately 50%,or between about 5% and 95%) of the incident light at the outputwavelength to the second harmonic generator 236. Light at the outputwavelength not transmitted by the output coupler 232 is reflected backinto the optical cavity. Mirror 230 serves to direct the output light inthe correct direction. In one embodiment, mirror 230 may not berequired. In another embodiment, multiple mirrors may be used instead ofmirror 230. In yet another embodiment, one or more prisms may be insteadof the mirror 230.

FIG. 2C illustrates a simplified block diagram of an alternativeexemplary embodiment of an 1109 nm generator 240 that can perform thefunctions of the 1109 nm generator 108 (FIG. 1A) and the 1109 nmgenerator 128 (FIG. 1B). In this embodiment, a 2218 nm seed laser 242 isused to generate a low-power signal of the desired wavelength andbandwidth that is input into an optical parametric amplifier (OPA) 243along with a portion of a fundamental light 241. The OPA 243 operates ina similar manner to the configuration described with respect to FIG. 2B,but it does not need a narrow-band wavelength selective element (such asa volume Bragg grating), because the 2218 nm seed laser 242 determinesthe wavelength and bandwidth. The OPA 243 may use a similar non-linearcrystal, such as LNB, lithium tantalate or KTP (bulk or periodicallypoled) as described above. The output of the OPA 243 is directed to asecond harmonic generator 246, which generates a desired 1109 nm light245. The second harmonic generator 246 may be configured similarly tothe second harmonic generator 236 (FIG. 2B).

FIG. 3 shows an illustrative block diagram of an exemplary embodiment ofa frequency mixer 300 that creates an output light 305 at a wavelengthnear 193 nm, such as a wavelength of substantially 193.4 nm. Frequencymixer 300 can perform the function of frequency mixer 110 (FIG. 1A) andof frequency mixer 130 (FIG. 1B). In this embodiment, a 234 nm light301, such as a wavelength near 234.2 nm, is mixed in a frequency mixerblock 304 with a 1109 nm light 302 to create the output light 305. Thefrequency mixer block 304 may include a non-linear crystal, such as CLBOor BBO as described above. In preferred embodiments, the non-linearcrystal is kept in a controlled environment to maintain a constanttemperature and protect the crystal from humidity and contaminants.Details of such protective environments can be found in U.S. Pat. No.8,298,335 by Armstrong, which issued on Oct. 30, 2012, and isincorporated by reference herein. In this embodiment, any unconsumedinput light 306 is separated from the output light 305 using prims,polarizing beam splitters, or other means.

FIG. 4A shows an illustrative block diagram of an exemplary embodimentof the 234 nm generator 400 that creates light 411 at a wavelength near234 nm, such as a wavelength of substantially 234.2 nm. The 234 nmgenerator 400 can perform the function of the frequency conversionstages 106 of FIG. 1A.

The 234 nm generator 400 uses a third-harmonic generator 402 to create athird harmonic 407 by combining a portion 401 of the fundamentalfrequency with the second harmonic 403. If the fundamental wavelength isclose to 1064 nm, then the third harmonic will have a wavelength closeto 355 nm. If the fundamental is close to 1053 nm, then the thirdharmonic will have a wavelength close to 351 nm. If the fundamental isclose to 1047 nm, then the third harmonic will have a wavelength closeto 349 nm. The third harmonic generator 402 includes a non-linearcrystal such as CLBO, BBO or LB4. The fundamental 401 and the secondharmonic 403 can be taken from the output of the second harmonicgenerator 104 shown in FIG. 1A.

Another portion 405 of the fundamental frequency is used by an opticalparametric amplifier or optical parametric oscillator 406 to generatelight 409 at a wavelength of approximately 689 nm. The light 409 at awavelength of approximately 689 nm is mixed with the third harmonic 407in the frequency mixer 408 to generate the output light 411 at awavelength near 234 nm. Unconsumed third harmonic and 689 nm light canbe separated from the output of the frequency mixer 408 and discarded as412. The portion 405 of the fundamental can be taken from the output ofthe second harmonic generator 104, from the output of the third harmonicgenerator 402, from the output of the 1109 nm generator 108, directlyfrom the fundamental laser 102, or any other convenient place.

The exact wavelength of the light 409 at approximately 689 nm should bechosen so as to generate the desired output wavelength at 411. Forexample, in preferred embodiments, the output wavelength 411 issubstantially 234.2 nm. In such embodiments, if, for example, thefundamental is close to 1064.4 nm, then the light 409 should have awavelength of substantially 689.0 nm. If the fundamental is close to1053.0 nm, then the light 409 should have a wavelength close to 703.8nm. If the fundamental is close to 1047.0 nm, then the light 409 shouldhave a wavelength close to 712.0 nm.

In some embodiments, a seed laser diode 404 at the desired wavelength ofapproximately 689 nm, such as a wavelength near 689.0, 703.8 or 712.0nm, with the desired bandwidth and stability is used to seed the opticalparametric amplifier or 406. In other embodiments, wavelength selectiveelements such as a volume Bragg grating, or a diffraction grating, isused to determine the center wavelength and bandwidth of the opticalparametric amplifier or optical parametric oscillator 406.

FIG. 4B shows an illustrative block diagram of an alternative exemplaryembodiment of the 234 nm generator 420 that creates light 431 at awavelength near 234 nm, such as a wavelength of substantially 234.2 nm.The 234 nm generator 420 can perform the function of the frequencyconversion stages 106 of FIG. 1A.

The 234 nm generator 420 creates a fourth harmonic 425 from the secondharmonic 421 in the frequency doubler 422. If the fundamental wavelengthis close to 1064 nm, then the fourth harmonic will have a wavelengthclose to 266 nm. If the fundamental is close to 1053 nm, then the fourthharmonic will have a wavelength close to 263.3 nm. If the fundamental isclose to 1047 nm, then the fourth harmonic will have a wavelength closeto 261.8 nm. The frequency doubler 422 includes a non-linear crystalsuch as CLBO, BBO or LB4. The second harmonic 421 can be taken from theoutput of the second harmonic generator 104 shown in FIG. 1A.

A portion 423 of the fundamental frequency is used by an opticalparametric amplifier or optical parametric oscillator 426 to generatelight 429 at a wavelength of approximately 1954 m. The light 429 at awavelength of approximately 1954 nm is mixed with the fourth harmonic425 in the frequency mixer 428 to generate the output light 431 at awavelength near 234 nm. Any unconsumed fourth harmonic and approximately1954 nm light can be separated from the output of the frequency mixer428 and discarded as 432. The portion 423 of the fundamental can betaken from the output of the second harmonic generator 104, from theoutput of the 1109 nm generator 108, directly from the fundamental laser102, or any other convenient place.

The exact wavelength of the light 429 at approximately 1954 nm should bechosen so as to generate the desired output wavelength at 431. Forexample, in preferred embodiments, the output wavelength 411 issubstantially 234.2 nm. In such embodiments, if, for example, thefundamental is close to 1064.4 nm, then the light 429 should have awavelength of substantially 1954 nm. If the fundamental is close to1053.0 nm, then the light 409 should have a wavelength close to 2122 nm.If the fundamental is close to 1047.0 nm, then the light 409 should havea wavelength close to 2225 nm.

In some embodiments, a seed laser diode 424 at the desired wavelength ofapproximately 1954 nm, such as a wavelength near 1954, 2122 or 2225 nm,with the desired bandwidth and stability is used to seed the opticalparametric amplifier or optical parametric oscillator 426. In otherembodiments, wavelength selective elements such as a volume Bragggrating, or a diffraction grating, is used to determine the centerwavelength and bandwidth of the optical parametric amplifier or opticalparametric oscillator 426.

FIG. 5A shows an illustrative block diagram of an exemplary embodimentof the 1171 nm generator 500 that creates light 509 at a wavelength near1171 nm. The 1171 nm generator 500 can perform the function of the 1171nm generator 124 of FIG. 1B or the 1171 nm generator 144 of FIG. 1C. The1171 nm generator 500 generates the output light 509 by amplifying seedlaser light 503 with desired center wavelength (near 1171 nm) andbandwidth. The amplification is performed by a Raman amplifier 506. TheRaman amplifier may comprise a fused silica fiber or may comprise agermania-doped fused silica fiber. A stable seed laser 502, such as afrequency-stabilized laser diode or low-power fiber laser generates theseed laser light 503. In some embodiments, the seed laser 502 maygenerate a power between about 1 mW and 250 mW. The seed laser 502 maybe a CW laser, or may be a pulsed laser that is synchronized with thefundamental laser. The seed laser light 503 is combined with the pumplaser light 501 by a wavelength combiner 504. The pump laser light 501may comprise light at a wavelength near 1109 nm or may comprise thefundamental wavelength and may, for example, be taken from the outputof, or unconsumed fundamental from, the 1109 nm generator 128 in FIG.1B, the 1109 nm generator 148 in FIG. 1C, or directly from thefundamental laser 122 (FIG. 1B) or 142 (FIG. 1C). The pump laser light501 may also be taken from unconsumed 1109 nm light 129′ from thefrequency mixer 130 as shown in FIG. 1B. As described above, thesecond-order Raman shift of fused silica may be efficiently used with apump wavelength near 1064 nm or near 1053 nm. The wavelength separator507 separates unconsumed pump laser light 511 from the output light 509.The unconsumed pump laser light 511 may be used as an input to anotherstage, or may be dumped.

FIG. 5B shows an illustrative block diagram of an alternative exemplaryembodiment of the 1171 nm generator 520 that creates light 529 at awavelength near 1171 nm. The 1171 nm generator 520 can perform thefunction of the 1171 nm generator 124 of FIG. 1B or the 1171 nmgenerator 144 of FIG. 1C. The 1171 nm generator 520 generates the outputlight 529 at a wavelength near 1171 nm using a fiber optical parametricoscillator that includes a Raman amplifier. The amplification isperformed by a Raman amplifier 526, which generates a first-order orsecond-order Raman shift from the pump wavelength in a fused silica orgemania-doped fused silica fiber in manner similar to that justdescribed for FIG. 5A. A portion 511 of the output wavelength near 1171nm is fed back by an output coupler 527. In preferred embodimentsbetween about 1% and about 50% of the output wavelength may be fed back.A narrow-band filter 528, such as a fiber Bragg grating, selects thewavelength and bandwidth to feedback and hence determines the wavelengthand bandwidth of the output. The portion 511 of the output light that isfed back is combined with the pump laser light 521 by the wavelengthcombiner 524. The pump laser light 501 is light at a wavelength ofapproximately 1109 nm or at the fundamental wavelength and may, forexample, be taken from the output of, or unconsumed fundamental from,the 1109 nm generator 128 in FIG. 1B, the 1109 nm generator 148 in FIG.1C, or directly from the fundamental laser 122 (FIG. 1B) or thefundamental laser 142 (FIG. 1C). As described above, the second-orderRaman shift of fused silica may be efficiently used with a pumpwavelength near 1064 nm or near 1053 nm. The output is a mixture of theoutput light at a wavelength near 1171 nm and unconsumed pumpwavelength. Those wavelengths may be separated if desired. The 1171 nmgenerator 520 can be built entirely from fiber-optic based components.This can be particularly advantageous if the fundamental laser is afiber laser.

FIG. 6A shows an illustrative block diagram of an exemplary preferredembodiment of the fifth harmonic generator 600 that creates output light607 at a wavelength near 234 nm, such as a wavelength of substantially234.2 nm, from the input light 601 at a wavelength of 1171 nm. Fifthharmonic generator 600 generates the fifth harmonic 607 by firstgenerating the second harmonic at a wavelength of 585.5 nm in a secondharmonic generator 602. The second harmonic generator 602 includes anon-linear crystal, preferably LBO, which is phase-matched forgenerating the second harmonic at an angle of about 83° at a temperatureof about 120° C. with a low walk-off angle of about 6 mrad. The output603 of the second harmonic generator 602 includes both unconsumed 1171nm light and the second harmonic at a wavelength of 585.5 nm.

The output 603 of the second harmonic generator 602 is passed to thethird harmonic generator 604 that creates the third harmonic by mixingthe 1171 nm wavelength with the second harmonic at 585.5 nm. The thirdharmonic generator 604 includes a non-linear crystal, CLBO in onepreferred embodiment, which is phase-matched for generating the thirdharmonic at an angle of about 77.5° at a temperature of about 120° C.with a walk-off angle of about 15 mrad. The output 605 of the thirdharmonic generator 604 includes unconsumed 1171 nm and 585.5 nm lightand the third harmonic at a wavelength close to 390.3 nm. Any unconsumed1171 nm light may be separated from the output or may be passed to thenext stage if it will not cause any problems.

The output 605 of the third harmonic generator 604 is passed to thefifth harmonic generator 606 that creates the fifth harmonic 607 bymixing the 585.5 nm wavelength second harmonic with the 390.3 nmwavelength third harmonic. The fifth harmonic generator 606 includes anon-linear crystal, preferably CLBO, which is phase-matched forgenerating the fifth harmonic at an angle of about 86.4° at atemperature of about 120° C. with a walk-off angle of about 5 mrad. Anyunconsumed 1171 nm, 585.5 nm or 390.3 nm light may be separated orfiltered from the output 607.

FIG. 6B shows an illustrative block diagram of an alternative exemplaryembodiment of the fifth harmonic generator 620 that creates output light627 at a wavelength near 234 nm, such as a wavelength of substantially234.2 nm from the input light 601 at a wavelength of 1171 nm. Fifthharmonic generator 620 generates the fifth harmonic 627 by firstgenerating the second harmonic at a wavelength of 585.5 nm in a secondharmonic generator 622 that functions substantially similarly to secondharmonic generator 602 of FIG. 6A. The unconsumed 1171 nm light 629 atthe output of the second harmonic generator 622 may be separated fromthe second harmonic 623 and directed to the fifth harmonic generator 626using, for example, mirrors and/or prisms such as those labeled 630. Insome embodiments, it may be possible to pass the unconsumed 1171 nmlight 629 through the fourth harmonic generator 624 because it is notphase matched and does not significantly interfere with the frequencyconversion.

The second harmonic 623 at a wavelength of 585.5 nm is passed to fourthharmonic generator 624. Fourth harmonic generator 624 includes anon-linear crystal such as CLBO, BBO or KDP (potassium dihydrogenphosphate). Fourth harmonic generator 624 creates the fourth harmonic625 at a wavelength of 292.8 nm. Unconsumed second harmonic may beseparated from the output of the fourth harmonic generator 624.

The fourth harmonic 625 is passed to the fifth harmonic generator 626which combines it with light at 1171 nm to create the fifth harmonicoutput 627 at a wavelength near 234.2 nm. Unconsumed light at 1171 nm or292.8 nm may be separated or filtered from the output. The fifthharmonic generator 626 includes a non-linear crystal such as KDP, CLBO,BBO or LB4.

In some embodiments, to generate sufficient power at the fundamentalwavelength, two or more amplifiers may be used. Note that if two or moreamplifiers are used, then one seed laser should preferably be used toseed all the amplifiers so that the outputs from all amplifiers are atthe same wavelength and are synchronized one with another. This isillustrated by the block diagram 700 in FIG. 7. Multiple amplifiers areadvantageous when a single amplifier cannot easily be made to operate atthe desired power level with the required bandwidth due to effects suchas thermal lensing, self-phase modulation, or cross-phase modulation, orin cases where the heat dissipated in a single amplifier makes coolingthat amplifier difficult or expensive.

The seed laser 703 generates light at the desired fundamental wavelengthwith the right bandwidth near 1064 nm, 1053 nm or 1047 nm. The seedlaser (or oscillator) may be a diode laser, an Nd-doped yttriumorthovanadate laser, an Nd:YAG laser, an Nd:YLF laser or a fiber laser.In one embodiment, the output of the seed laser 704 is divided by beamsplitter 711 and is directed to two or more amplifiers such as 707 and717. Each amplifier outputs light (701 and 711 respectively) at thefundamental wavelength, but at a higher power than the output of theseed laser 703. Mirrors and/or prisms such as 712 may be used as neededto direct the fundamental seed light 704 to the different amplifierssuch as 707 and 717. Each amplifier has its own pump (shown as 705 and715), which, preferably, comprises laser diodes.

Any of the harmonic generators or frequency mixers may use some, or all,of the methods and systems disclosed in U.S. Pat. No. 8,873,596,entitled “Laser With High Quality, Stable Output Beam, And Long LifeHigh Conversion Efficiency Non-Linear Crystal”, by Dribinski et al.,issued Oct. 28, 2014 and incorporated by reference herein. Any of theharmonic generators or frequency mixers, particularly those generatingUV wavelengths, may advantageously use hydrogen-annealed non-linearcrystals. Such crystals may be processed as described in U.S. Pat. No.9,250,178, entitled “Hydrogen Passivation of Nonlinear Optical Crystals”by Chuang et al., issued Feb. 2, 2016, which is incorporated byreference herein.

Any of the frequency conversion, harmonic generation, or frequencymixing stages may be in a protected environment, such as the protectedenvironment described in the '335 patent. This protected environment isparticularly useful in protecting stages that use or generatewavelengths shorter than about 300 nm, since such wavelengths can easilycause photocontamination of optical surfaces. The protected environmentis also very useful for stages that include a hygroscopic material suchas CLBO, LBO or BBO. A single protective environment may protect justone stage, or may protect multiple stages.

As known by those skilled in the art, mirrors, or prisms may be used todirect the light where needed. Lenses and curved mirrors may be used tofocus the beam waist to a point inside or proximate to the non-linearcrystals where appropriate. Prisms, gratings, beam splitters, ordiffractive optical elements may be used to separate the differentwavelengths at the outputs of each harmonic generator module whenneeded. Prisms, beam splitters, diffractive optical elements, ordichroic mirrors may be used to combine wavelengths where needed. Beamsplitters or coated mirrors may be used as appropriate to divide onewavelength into two beams.

Note that these techniques and additional details are exemplary and anylaser constructed in accordance with this application may vary based onimplementation and/or system constraints. Multiple embodiments aredescribed above illustrating several variations and equivalents of thisapproach for generating light near 193 nm. When a sub-200 nm wavelengthis required, such as a wavelength in the range from approximately 190 nmto approximately 200 nm, but not substantially 193.4 nm, small changescould be made to one or more of the wavelengths generated by opticalparametric or Raman shift amplifiers without departing from the scope ofthis invention. One skilled in the relevant arts will appreciate thatdifferent, but substantially equivalent, frequency conversion techniquesmay be used without departing from the scope of the invention. Anyembodiment might use multiple crystals in a walkoff-compensationgeometry to improve the frequency conversion efficiency and beam profilein any critically phase matched stage.

FIG. 8 illustrates an exemplary pulse multiplier 8120 that may be usedwith any of the above described laser embodiments to increase the pulserepetition rate in a metrology or inspection system. Increasing therepetition rate of the fundamental laser while maintaining the pulsewidth and maintaining constant average output power would result inreduced peak power and, hence, lower efficiency from the frequencyconversion and mixing stages. The pulse multiplier 8120 overcomes thisproblem by leaving the fundamental laser repetition rate unchanged anddividing each output pulse into multiple pulses, thus increasing therepetition rate without reducing the efficiency of the frequencyconversion and mixing stages.

Pulse multiplier 8120 is configured to generate pulse trains from eachinput pulse. Input pulses at a wavelength of approximately 193 nm arrivefrom direction 8121 and impinge on a beam splitter 8123, which reflectspart of each pulse in an output direction 8122, and transmits part intoa ring cavity towards a mirror 8126. The input and output pulses aresubstantially polarized in a direction parallel to the arrow 8124. Thus,the output polarization is substantially parallel to the inputpolarization.

The ring cavity includes a mirror 8126, a prism 8128, and the beamsplitter 8123. The mirror 8126 refocuses the light circulating withinthe ring cavity. Preferably, the radius of curvature of the mirror 8126is substantially equal to half of the optical path length of the ringcavity so that the beam waist is refocused with a magnification of oneeach trip around the ring cavity. Brewster's angle cuts are preferablyused for the input and output faces of the prism 8128, therebyminimizing or largely eliminating reflection losses at those faces (theinput face of prism 8128 is labeled 8129) because the light incident onthe face of the prism 8128 is substantially p polarized relative to thatface. After light exits the prism 8128, it is directed back to the beamsplitter 8123, where part of each pulse is transmitted through the beamsplitter 8123 in the output direction 8122, and part is reflected backinto the ring cavity.

Details of this pulse multiplier and alternative pulse multiplierconfigurations are described in co-owned U.S. Pat. No. 9,151,940,entitled “SEMICONDUCTOR INSPECTION AND METROLOGY SYSTEM USING LASERPULSE MULTIPLIER”, by Chuang et al., issued Oct. 6, 2015 and claimingpriority to U.S. Provisional Application 61/733,858, entitled“Semiconductor Inspection And Metrology System Using Laser PulseMultiplier”, and filed on Dec. 5, 2012, and in co-pending U.S. PublishedApplication 2012/0314286 entitled “Semiconductor Inspection AndMetrology System Using Laser Pulse Multiplier”, published on Dec. 13,2012 by Chuang et al. and claiming priority to U.S. ProvisionalApplication 61/496,446, filed on Jun. 13, 2011 by Chuang et al. Thesepatents and patent applications are incorporated by reference herein.

As described in the '593 application, the optical path length of thering cavity may be set to be approximately equal to an integer fractionof the distance between successive incoming pulses, where the distancebetween two pulses is equal to the velocity of light multiplied by thetime interval between those pulses. For example, in some embodiments theoptical path length of the cavity may be set to be approximately onehalf of the distance between the incoming pulses. For such a ringcavity, every second pulse will approximately coincide with an arrivinginput pulse, thus doubling the repetition rate. The '593 applicationalso describes how the optical cavity length may be set slightly longeror slightly shorter than half of the distance between incoming pulses soas to further reduce the peak power of the output pulses.

The '593 application describes how, in preferred embodiments, the beamsplitter 8123 reflects approximately one third of the energy of eachincident pulse and transmits approximately two thirds of the energy ofeach incident pulse so as to generate an output stream of substantiallyequal energy pulses in a pulse rate doubler. This application furtherdescribes how to adjust the transmission and reflection ratios of thebeam splitter 8123 in order to achieve substantially equal output pulseenergies in the presence of beam splitter and cavity losses.

FIG. 9 illustrates aspects of a pulse-shaping or coherence reducingdevice used in conjunction with a pulsed laser, suitable forincorporation into an inspection or metrology system in accordance withembodiments of the present invention. A light source 910 comprises a 193nm or sub-200 nm laser as described herein. The light source 910generates a light beam 912 comprising a series of pulses. One aspect ofthis embodiment is to make use of the finite spectral range of the laserin order to perform a substantially quick temporal modulation of a lightbeam 912, which can be changed on approximately one-tenth-picosecondtime scales (a tenth picosecond time interval is equivalent to about 1pm in spectral width for a wavelength near 193 nm), and transform thetemporal modulation to spatial modulation.

The use of a dispersive element and an electro-optic modulator isprovided for speckle reduction and/or pulse shaping. For example, theillumination subsystem includes a dispersive element positioned in thepath of the coherent pulses of light. As shown in FIG. 9, the dispersiveelement can be positioned at a plane 914 arranged at angle θ₁ to thecross-section x₁ of the coherent pulses of light. As further shown inFIG. 9, the pulses of light exit the dispersive element at angle θ₁′ andwith cross-sectional dimension x₁′. In one embodiment, the dispersiveelement is a prism. In another embodiment, the dispersive element is adiffraction grating. The dispersive element is configured to reducecoherence of the pulses of light by mixing spatial and temporalcharacteristics of light distribution in the pulses of light. Inparticular, a dispersive element such as a prism or diffraction gratingprovides some mixing between spatial and temporal characteristics of thelight distribution in the pulses of light. The dispersive element mayinclude any suitable prism or diffraction grating, which may varydepending on the optical characteristics of the illumination subsystemand the metrology or inspection system.

The illumination subsystem further includes an electro-optic modulatorpositioned in the path of the pulses of light exiting the dispersiveelement. For example, as shown in FIG. 9, the illumination subsystem mayinclude an electro-optic modulator 916 positioned in the path of thepulses of light exiting the dispersive element. The electro-opticmodulator is configured to reduce the coherence of the pulses of lightby temporally modulating the light distribution in the pulses of light.In particular, the electro-optic modulator provides an arbitrarytemporal modulation of the light distribution. Therefore, the dispersiveelement and the electro-optic modulator have a combined effect on thepulses of light generated by the light source. In particular, thecombination of the dispersive element with the electro-optic modulatorcreates an arbitrary temporal modulation and transforms the temporalmodulation to an arbitrary spatial modulation of an output beam 918.

In one embodiment, the electro-optic modulator is configured to changethe temporal modulation of the light distribution in the pulses of lightat tenth picosecond time intervals. In another embodiment, theelectro-optic modulator is configured to provide about 1000 aperiodicsamples on each period of the modulation of the electro-optic modulatorthereby providing a de-coherence time of about 10⁻¹³ seconds.

Further details of pulse-shaping and coherence and speckle reducingdevices suitable for use in conjunction with a sub-200 nm laser in aninspection or metrology system can be found in U.S. Published PatentApplications 2011/0279819, entitled “Illumination Subsystems Of AMetrology System, Metrology Systems, and Methods For Illuminating ASpecimen For Metrology Measurements” published on Nov. 17, 2011, and2011/0228263, entitled “Illuminating A Specimen For Metrology OrInspection” published on Sep. 22, 2011, both by Chuang et al. Both ofthese applications are incorporated by reference herein.

FIGS. 10-15 illustrate systems that can include the above-described 193nm or sub-200 nm lasers. These systems can be used in photomask,reticle, or wafer inspection applications.

In accordance with certain embodiments of the present invention, aninspection system that incorporates a 193 nm or sub-200 nm laser maysimultaneously detect two channels of data on a single detector. Such aninspection system may be used to inspect a substrate such as a reticle,a photomask or a wafer, and may operate as described in U.S. Pat. No.7,528,943 by Brown et al., issued on May 15, 2009, and is incorporatedby reference herein.

FIG. 10 shows a reticle, photomask, or wafer inspection system 1000 thatsimultaneously detects two channels of image or signal on one sensor1070. An illumination source 1009 can include a 193 nm or sub-200 nmlaser as described herein. The illumination source 1009 may furthercomprise a pulse multiplier and/or a coherence reducing scheme. The twochannels may comprise reflected and transmitted intensity when aninspected object 1030 is transparent (for example a reticle orphotomask), or may comprise two different illumination modes, such asangles of incidence, polarization states, wavelength ranges, or somecombination thereof.

As shown in FIG. 10, illumination relay optics 1015 and 1020 relay theillumination from illumination source 1009 to the inspected object 1030.The inspected object 1030 may be a reticle, a photomask, a semiconductorwafer, or other article to be inspected. Image relay optics 1055 and1060 relay the light that is reflected and/or transmitted by theinspected object 1030 to the sensor 1070. The data corresponding to thedetected signals or images for the two channels is shown as data 1080and is transmitted to a computer (not shown) for processing.

Other details of reticle and photomask inspection systems and methodsthat may be configured to measure transmitted and reflected light from areticle or photomask are described in U.S. Pat. No. 7,352,457 to Kvammeet al, which issued Apr. 1, 2008, and in U.S. Pat. No. 5,563,702 toEmery et al, which issued Oct. 8, 1996, both of which are incorporatedby reference herein.

FIG. 11 illustrates an exemplary inspection system 1100 includingmultiple objectives and one of the above-described lasers operating at awavelength near 193 nm, such as at wavelength between about 190 nm and200 nm. In system 1100, illumination from a laser source 1101 is sent tomultiple sections of the illumination subsystem. A first section of theillumination subsystem includes elements 1102 a through 1106 a. Lens1102 a focuses light from laser 1101. Light from lens 1102 a thenreflects from mirror 1103 a. Mirror 1103 a is placed at this locationfor the purposes of illustration, and may be positioned elsewhere. Lightfrom mirror 1103 a is then collected by lens 1104 a, which formsillumination pupil plane 1105 a. An aperture, filter, or other device tomodify the light may be placed in pupil plane 1105 a depending on therequirements of the inspection mode. Light from pupil plane 1105 a thenpasses through lens 1106 a and forms illumination field plane 1107.

A second section of the illumination subsystem includes elements 1102 bthrough 1106 b. Lens 1102 b focuses light from laser 1101. Light fromlens 1102 b then reflects from mirror 1103 b. Light from mirror 1103 bis then collected by lens 1104 b which forms illumination pupil plane1105 b. An aperture, filter, or other device to modify the light may beplaced in pupil plane 1105 b depending on the requirements of theinspection mode. Light from pupil plane 305 b then passes through lens1106 b and forms illumination field plane 1107. Illumination field lightenergy at illumination field plane 1107 is thus comprised of thecombined illumination sections.

Field plane light is then collected by lens 1109 before reflecting offbeamsplitter 1110. Lenses 1106 a and 1109 form an image of firstillumination pupil plane 1105 a at objective pupil plane 1111. Likewise,lenses 1106 b and 1109 form an image of second illumination pupil plane1105 b at objective pupil plane 1111. An objective 1112 (oralternatively 1113) then takes pupil light 1111 and forms an image ofillumination field 1107 at the sample 1114. Objective 1112 or 1113 canbe positioned in proximity to sample 1114. Sample 1114 can move on astage (not shown), which positions the sample in the desired location.Light reflected and scattered from the sample 1114 is collected by thehigh NA catadioptric objective 1112 or objective 1113. After forming areflected light pupil at point 1111, light energy passes beamsplitter1110 and lens 1115 before forming an internal field 1116 in the imagingsubsystem. This internal imaging field is an image of sample 1114 andcorrespondingly illumination field 1107. This field may be spatiallyseparated into multiple fields corresponding to the illumination fields.Each of these fields can support a separate imaging mode. For example,one imaging mode may be a bright-field imaging mode, while another maybe a dark-field imaging mode.

One of these fields can be redirected using mirror 1117. The redirectedlight then passes through lens 1118 b before forming another imagingpupil 1119 b. This imaging pupil is an image of pupil 1111 andcorrespondingly illumination pupil 1105 b. An aperture, filter, or otherdevice to modify the light may be placed in pupil plane 1119 b dependingon the requirements of the inspection mode. Light from pupil plane 1119b then passes through lens 1120 b and forms an image on sensor 1121 b.In a similar manner, light passing by mirror or reflective surface 1117is collected by lens 1118 a and forms imaging pupil 1119 a. Light fromimaging pupil 1119 a is then collected by lens 1120 a before forming animage on detector 1121 a. Light imaged on detector 1121 a can be usedfor a different imaging mode from the light imaged on sensor 1121 b.

The illumination subsystem employed in system 1100 is composed of lasersource 1101, collection optics 1102-1104, beam shaping components placedin proximity to a pupil plane 1105, and relay optics 1106 and 1109. Aninternal field plane 1105 is located between lenses 1106 and 1109. Inone preferred configuration, laser source 1101 can include one of theabove-described 193 nm or sub-200 nm lasers.

With respect to laser source 1101, while illustrated as a single uniformblock having two outputs, in reality this represents a laser source ableto provide two channels of illumination, for example a first channel oflight energy such as laser light energy at a first frequency (forexample, a wavelength close to 193 nm) which passes through elements1102 a-1106 a, and a second channel of light energy such as laser lightenergy at a second frequency (for example, a wavelength close to 234 nm)which passes through elements 1102 b-1106 b. Different illumination anddetection modes may be employed, such as a bright-field mode in onechannel and a dark-field mode in the other channel.

While light energy from laser source 1101 is shown to be emitted 90degrees apart, and the elements 1102 a-1106 a and 1102 b-1106 b areoriented at 90 degree angles, in reality light may be emitted at variousorientations, not necessarily in two dimensions, and the components maybe oriented differently than as shown. FIG. 11 is therefore simply arepresentation of the components employed and the angles or distancesshown are not to scale nor specifically required for the design.

Elements placed in proximity to pupil plane 1105 may be employed in thecurrent system using the concept of aperture shaping. Using this design,uniform illumination or near uniform illumination may be realized, aswell as individual point illumination, ring illumination, quadrapoleillumination, or other desirable patterns.

Various implementations for the objectives may be employed in a generalimaging subsystem. A single fixed objective may be used. The singleobjective may support all the desired imaging and inspection modes. Sucha design is achievable if the imaging system supports a relatively largefield size and relatively high numerical aperture. The numericalaperture can be reduced to a desired value by using internal aperturesplaced at the pupil planes 1105 a, 1105 b, 1119 a, and 1119 b.

Multiple objectives may also be used. For example, although twoobjectives 1112 and 1113 are shown, any number is possible. Eachobjective in such a design may be optimized for each wavelength producedby laser source 1101. These objectives can either have fixed positionsor be moved into position in proximity to the sample 1114. To movemultiple objectives in proximity to the sample, rotary turrets may beused as are common on standard microscopes. Other designs for movingobjectives in proximity of a sample are available, including but notlimited to translating the objectives laterally on a stage, andtranslating the objectives on an arc using a goniometer. In addition,any combination of fixed objectives and multiple objectives on a turretcan be achieved in accordance with the present system.

The maximum numerical apertures of the current embodiments approach orexceed 0.97, but may in certain instances be smaller. The wide range ofillumination and collection angles possible with this high NAcatadioptric imaging system, combined with its large field size allowsthe system to simultaneously support multiple inspection modes. As maybe appreciated from the previous paragraphs, multiple imaging modes canbe implemented using a single optical system or machine in connectionwith the illumination device. The high NA disclosed for illumination andcollection permits the implementation of imaging modes using the sameoptical system, thereby allowing optimization of imaging for differenttypes of defects or samples.

The imaging subsystem also includes intermediate image forming optics1115. The purpose of the image forming optics 1115 is to form aninternal image 1116 of the sample 1114. At this internal image 1116, amirror 1117 can be placed to redirect light corresponding to one of theinspection modes. It is possible to redirect the light at this locationbecause the light for the imaging modes are spatially separate. Theimage forming optics 1118 and 1120 can be implemented in severaldifferent forms including a varifocal zoom, multiple afocal tube lenseswith focusing optics, or multiple image forming mag tubes. U.S.Published Patent Application 2009/0180176, which published on Jul. 16,2009 and is incorporated by reference herein, describes additionaldetails of system 1100.

FIG. 12 illustrates the addition of a normal incidence laser dark-fieldillumination to a catadioptric imaging system 1200. The dark-fieldillumination includes a sub-200 nm laser 1201, adaptation optics 1202 tocontrol the illumination beam size and profile on the surface beinginspected, an aperture and window 1203 in a mechanical housing 1204, anda prism 1205 to redirect the laser along the optical axis at normalincidence to the surface of a sample 1208. Prism 1205 also directs thespecular reflection from surface features of sample 1208 and reflectionsfrom the optical surfaces of an objective 1206 along the optical path toan image plane 1209. Lenses for objective 1206 can be provided in thegeneral form of a catadioptric objective, a focusing lens group, and azooming tube lens section (see U.S. Pat. No. 5,999,310, which issued onDec. 7, 1999 and is incorporated by reference herein). In a preferredembodiment, laser 1201 can include the above-described 193 nm or sub-200nm laser. In some embodiments, the laser 1201 may further include theabove described pulse multiplier and/or the above described coherencereducer. U.S. Published Patent Application 2007/0002465, which publishedon Jan. 4, 2007 and is incorporated by reference herein, describessystem 1200 in further detail.

FIG. 13A illustrates a surface inspection apparatus 1300 that includesillumination system 1301 and collection system 1310 for inspecting areasof surface 1311. As shown in FIG. 13A, a laser system 1320 directs alight beam 1302 through a lens 1303. In a preferred embodiment, lasersystem 1320 includes the above-described sub-200 nm laser, an annealedcrystal, and a housing to maintain the annealed condition of thecrystal. First beam shaping optics can be configured to receive a beamfrom the laser and focus the beam to an elliptical cross section at abeam waist in or proximate to the crystal.

Lens 1303 is oriented so that its principal plane is substantiallyparallel to a sample surface 1311 and, as a result, illumination line1305 is formed on surface 1311 in the focal plane of lens 1303. Inaddition, light beam 1302 and focused beam 1304 are directed at anon-orthogonal angle of incidence to surface 1311. In particular, lightbeam 1302 and focused beam 1304 may be directed at an angle betweenabout 1 degree and about 85 degrees from a normal direction to surface1311. In this manner, illumination line 1305 is substantially in theplane of incidence of focused beam 1304.

Collection system 1310 includes lens 1312 for collecting light scatteredfrom illumination line 1305 and lens 1313 for focusing the light comingout of lens 1312 onto a device, such as charge coupled device (CCD) orCMOS sensor 1314, comprising an array of light sensitive detectors. Inone embodiment, sensor 1314 may include a linear array of detectors. Insuch cases, the linear array of detectors within CCD or CMOS sensor 1314can be oriented parallel to illumination line 1315. In one embodiment,multiple collection systems can be included, wherein each of thecollection systems includes similar components, but differ inorientation.

For example, FIG. 13B illustrates an exemplary array of collectionsystems 1331, 1332, and 1333 for a surface inspection apparatus (whereinits illumination system, e.g. similar to that of illumination system1301, is not shown for simplicity). First optics in collection system1331 can collect a first beam of radiation along a first path from aline on the surface of sample 1311. Second optics in collection system1332 can collect a second beam of radiation along a second path from thesame line on the surface of sample 1311. Third optics in collectionsystem 1333 can collect a third beam of radiation along a third pathfrom the same line on the surface of sample 1311. Note that the first,second, and third paths are at different angles of incidence to saidsurface of sample 1311. A platform 1335 supporting sample 1311 can beused to cause relative motion between the multiple beams and sample 1311so that the line is scanned across the surface of sample 1311. U.S. Pat.No. 7,525,649, which issued on Apr. 28, 2009 and is incorporated byreference herein, describes surface inspection apparatus 1300 and othermultiple collection systems in further detail.

FIG. 14 illustrates an exemplary surface inspection system 1400 that canbe used for inspecting anomalies on a surface 1401. In this embodiment,surface 1401 can be illuminated by a substantially stationaryillumination device portion of a laser system 1430 comprising a laserbeam generated by the above-described 193 nm or sub-200 nm laser. Theoutput of laser system 1430 can be consecutively passed throughpolarizing optics 1421, a beam expander and aperture 1422, andbeam-forming optics 1423 to expand and focus the beam.

The focused laser beam 1402 is then reflected by a beam foldingcomponent 1403 and a beam deflector 1404 to direct the beam 1405 towardssurface 1401 for illuminating the surface. In the preferred embodiment,beam 1405 is substantially normal or perpendicular to surface 1401,although in other embodiments beam 1405 may be at an oblique angle tosurface 1401.

In one embodiment, beam 1405 is substantially perpendicular or normal tosurface 1401 and beam deflector 1404 reflects the specular reflection ofthe beam from surface 1401 towards beam turning component 1403, therebyacting as a shield to prevent the specular reflection from reaching thedetectors. The direction of the specular reflection is along line SR,which is normal to the surface 1401 of the sample. In one embodimentwhere beam 1405 is normal to surface 1401, this line SR coincides withthe direction of illuminating beam 1405, where this common referenceline or direction is referred to herein as the axis of inspection system1400. Where beam 1405 is at an oblique angle to surface 1401, thedirection of specular reflection SR would not coincide with the incomingdirection of beam 1405; in such instance, the line SR indicating thedirection of the surface normal is referred to as the principal axis ofthe collection portion of inspection system 1400.

Light scattered by small particles is collected by mirror 1406 anddirected towards aperture 1407 and detector 1408. Light scattered bylarge particles is collected by lenses 1409 and directed towardsaperture 1410 and detector 1411. Note that some large particles willscatter light that is also collected and directed to detector 1407, andsimilarly some small particles will scatter light that is also collectedand directed to detector 1411, but such light is of relatively lowintensity compared to the intensity of scattered light the respectivedetector is designed to detect. In one embodiment, detector 1411 caninclude an array of light sensitive elements, wherein each lightsensitive element of the array of light sensitive elements is configuredto detect a corresponding portion of a magnified image of theillumination line. In one embodiment, inspection system 1400 can beconfigured for use in detecting defects on unpatterned wafers. U.S. Pat.No. 6,271,916, which issued on Aug. 7, 2011 and is incorporated byreference herein, describes inspection system 1400 in further detail.

FIG. 15 illustrates another exemplary inspection system 1500 configuredto implement anomaly detection using both normal and obliqueillumination beams. In this configuration, a laser system 1530, whichincludes the above-described sub-200 nm laser, can provide a laser beam1501. A lens 1502 focuses the beam 1501 through a spatial filter 1503and lens 1504 collimates the beam and conveys it to a polarizing beamsplitter 1505. Beam splitter 1505 passes a first polarized component tothe normal illumination channel and a second polarized component to theoblique illumination channel, where the first and second components areorthogonal. In the normal illumination channel 1506, the first polarizedcomponent is focused by optics 1507 and reflected by mirror 1508 towardsa surface of a sample 1509. The radiation scattered by sample 1509 iscollected and focused by a paraboloidal mirror 1510 to a photomultipliertube or detector 1511.

In the oblique illumination channel 1512, the second polarized componentis reflected by beam splitter 1505 to a mirror 1513 which reflects suchbeam through a half-wave plate 1514 and focused by optics 1515 to sample1509. Radiation originating from the oblique illumination beam in theoblique channel 1512 and scattered by sample 1509 is collected byparaboloidal mirror 1510 and focused to photomultiplier tube 1511.Photomultiplier tube 1511 has a pinhole entrance. The pinhole and theilluminated spot (from the normal and oblique illumination channels onsurface 1509) are preferably at the foci of the paraboloidal mirror1510.

The paraboloidal mirror 1510 collimates the scattered radiation fromsample 1509 into a collimated beam 1516. Collimated beam 1516 is thenfocused by an objective 1517 and through an analyzer 1518 to thephotomultiplier tube 1511. Note that curved mirrored surfaces havingshapes other than paraboloidal shapes may also be used. An instrument1520 can provide relative motion between the beams and sample 1509 sothat spots are scanned across the surface of sample 1509. U.S. Pat. No.6,201,601, which issued on Mar. 13, 2001 and is incorporated byreference herein, describes inspection system 1500 in further detail.

The most critical frequency conversion step of a deep-UV laser is thefinal conversion stage. In the above-described lasers, this finalconversion stage mixes a wavelength of approximately 1109 nm with one ofapproximately 234 nm. CLBO enables the use of substantially non-criticalphase matching for that final frequency conversion with a phase matchingangle of approximately 85° at a temperature of approximately 80-120° C.Near non-critical phase matching is more efficient and more stable thancritical phase matching because the low walk-off angle (approximately7-9 mrad) allows a longer crystal to be used. Near non-critical phasematching is also less affected by small changes in alignment thancritical phase matching. Note that the longer crystal also allows theuse of lower peak power densities in the crystal while maintaining thesame overall conversion efficiency, thereby slowing damage accumulationto the crystal. Notably, mixing wavelengths of approximately 1109 nm andapproximately 234 nm is more efficient than 8^(th) harmonic generation.Therefore, the above-described 193 nm and sub-200 nm lasers can providesignificant system advantages for photomask, reticle, or waferinspection.

The various embodiments of the structures and methods of this inventionthat are described above are illustrative only of the principles of thisinvention and are not intended to limit the scope of the invention tothe particular embodiments described. For example, non-linear crystalsother than those listed above can be used for some of the frequencyconversion stages. Thus, the invention is limited only by the followingclaims and their equivalents.

The invention claimed is:
 1. A laser for generating sub-200 nmwavelength light, the laser comprising: a fundamental laser configuredto generate a fundamental wavelength between about 1030 nm and 1065 nm;a plurality of harmonic generators configured to convert the fundamentalwavelength into first light having a wavelength of approximately 1109 nmand second light having a wavelength of approximately 234 nm; and afrequency mixing stage configured to combine said first light at saidwavelength of approximately 1109 nm with said second light at saidwavelength of approximately 234 nm to generate said sub-200 nmwavelength light at a wavelength between 190 nm and 200 nm.
 2. The laserof claim 1, wherein the plurality of harmonic generators and thefrequency mixing stage are configured such that said sub-200 nmwavelength light has a wavelength of substantially 193.4 nm.
 3. Thelaser of claim 1, wherein the plurality of harmonic generators comprisesa first generator configured to generate said first light at saidwavelength of approximately 1109 nm by Raman shifting the fundamentalwavelength.
 4. The laser of claim 3, wherein the first generatorcomprises a Raman amplifier including one of an undoped fused-silicafiber and a Germania-doped fused silica fiber.
 5. The laser of claim 1,wherein the plurality of harmonic generators comprises a secondgenerator including: one of an optical parametric oscillator and anoptical parametric amplifier that is pumped by the fundamentalwavelength and configured to generate double-wavelength light having awavelength of approximately 2218 nm; and a second harmonic generatorconfigured to receive the double-wavelength light and to generate saidfirst light having said wavelength of approximately 1109 nm.
 6. Thelaser of claim 1, wherein said frequency mixing stage includes a CLBO(cesium lithium borate) crystal.
 7. The laser of claim 1, wherein saidfrequency mixing stage includes a non-linear optical crystal that hasbeen annealed in a hydrogen environment.
 8. The laser of claim 1,wherein the plurality of harmonic generators comprises a third generatorincluding: a third harmonic generator configured to generate a thirdharmonic of the fundamental wavelength; and a first frequency mixerconfigured to generate said second light at said wavelength ofapproximately 234 nm by mixing the third harmonic of the fundamentalwavelength with light at a wavelength of approximately 689 nm.
 9. Thelaser of claim 1, wherein the plurality of harmonic generators comprisesa fourth generator including: a fourth harmonic generator configured togenerate a fourth harmonic of the fundamental wavelength; and a secondfrequency mixer configured to generate said second light at saidwavelength of approximately 234 nm by mixing the fourth harmonic of thefundamental with light at a wavelength of approximately 1954 nm.
 10. Thelaser of claim 1, wherein the plurality of harmonic generatorscomprises: a fifth generator configured to generate third light at awavelength of approximately 1171 nm; and a fifth harmonic generatorconfigured to generate said second light at said wavelength ofapproximately 234 nm from said third light at said wavelength ofapproximately 1171 nm.
 11. The laser of claim 10, wherein the fifthgenerator comprises a fiber optical parametric oscillator configured toreceive a pump laser light having a pump wavelength determined by one ofthe first light and said fundamental frequency, said fifth generatorincluding a Raman amplifier configured to generate one of a first-orderRaman shift and a second-order Raman shift from the pump wavelength. 12.The laser of claim 11, wherein the fifth harmonic generator comprises: asecond harmonic generator configured to receive said third light at saidwavelength of approximately 1171 nm and to generate a second harmonic; athird harmonic generator configured to receive said second harmonic andto generate a third harmonic; and a fifth harmonic generator configuredto receive said third harmonic and to generate said second light at saidwavelength of approximately 234 nm.
 13. The laser of claim 1, whereinplurality of harmonic generators are configured such that the secondlight at said wavelength of approximately 234 nm is at a wavelength ofsubstantially 234.2 nm.
 14. The laser of claim 1, wherein thefundamental laser comprises one of an ytterbium-doped fiber laser, aneodymium-doped yttrium-aluminum-garnate laser, a neodymium-dopedyttrium-orthovanadate laser, and a neodymium-dopedyttrium-lithium-fluoride laser.
 15. A method of generating sub-200 nmwavelength laser light, the method comprising: generating a fundamentalwavelength between about 1030 nm and 1065 nm; using the fundamentalwavelength to generate first light having a wavelength of approximately1109 nm; using the fundamental wavelength to generate second lighthaving a wavelength of approximately 234 nm; combining the first lighthaving the wavelength of approximately 1109 nm and the second lighthaving the wavelength of approximately 234 nm to generate said sub-200nm wavelength laser light having a wavelength between 190 nm and 200 nm.16. The method of claim 15, wherein generating the first light havingthe wavelength of approximately 1109 nm comprises Raman shifting thefundamental wavelength.
 17. The method of claim 16, wherein the methodfurther comprises amplifying the first light having the wavelength ofapproximately 1109 nm using one of a fiber-optic amplifier, adoped-fiber-optic amplifier, a Germania-doped silica fiber Ramanamplifier, and an undoped silica fiber Raman amplifier.
 18. The methodof claim 15, wherein generating the first light having the wavelength ofapproximately 1109 nm comprises frequency doubling an output of one ofan optical parametric oscillator and optical parametric amplifier thatis pumped by the fundamental wavelength.
 19. The method of claim 15,wherein combining the first light having the wavelength of approximately1109 nm and the second light having the wavelength of approximately 234nm comprises mixing said first light and said second light in a CLBO(cesium lithium borate) crystal.
 20. The method of claim 15, whereincombining the first light having the wavelength of approximately 1109 nmand the second light having the wavelength of approximately 234 nmcomprises mixing said first light and said second light in a non-linearoptical crystal that has been annealed in a hydrogen environment. 21.The method of claim 15, wherein generating the second light having thewavelength of approximately 234 nm comprises mixing a third harmonic ofthe fundamental wavelength with light at a wavelength of approximately689 nm.
 22. The method of claim 15, wherein generating the second lighthaving the wavelength of approximately 234 nm comprises mixing a fourthharmonic of the fundamental wavelength with light at a wavelength ofapproximately 1954 nm.
 23. The method of claim 15, wherein generatingthe second light having the wavelength of approximately 234 nm comprisescreating a fifth harmonic of third light having a wavelength ofapproximately 1171 nm.
 24. The method of claim 15, wherein generatingthe fundamental wavelength comprises using one of an ytterbium-dopedfiber laser, a neodymium-doped yttrium-aluminum-garnate laser, aneodymium-doped yttrium-orthovanadate laser, and a neodymium-dopedyttrium-lithium-fluoride laser.